1. Field of the Invention
The present invention relates to a plastic ferrule capable of being used in an optical connector. The present invention further relates to a method of manufacturing a plastic ferrule, and also to a mold capable of being used in the manufacturing method.
2. Description of the Related Art
In recent years, in the field of optical connectors, a plastic ferrule has been developed, which is integrally molded in one piece from a resinous material, in view of promoting mass production and cost reduction. Conventionally, a plastic ferrule is comprised of a cylindrical centering section which has an uncoated optical fiber exposing port at one axial end face thereof, and a flange section projecting radially outward adjacent to the proximal end of the centering section. The centering section is provided, along its center axis, with an uncoated-fiber holding bore which opens to the fiber exposing port for securely accommodating an uncoated optical fiber having a coating removed, and a coated-fiber holding bore for securely accommodating a coated optical fiber with a coating, which has a diameter larger than the uncoated-fiber holding bore and communicates with the uncoated-fiber holding bore in axial direction. The flange section is formed integrally with the centering section as a unitary body for receiving a biasing force applied in an axially forward direction by a spring in the optical connector.
Usually, an optical connector includes a cylindrical positioning member (called, e.g., a split sleeve), and is constructed to concentrically connect a pair of optical fibers with each other, by axially aligning centering sections of a pair of ferrules, respectively fitted with optical fibers, with each other in one split sleeve, and by abutting the end faces of the centering sections to each other under the biasing force of a spring. In this condition, the split sleeve is expanded by the centering sections of the ferrules and exerts an elastic restoring force, thereby centering and holding both ferrules in a predetermined position under the restoring force. Therefore, the cylindrical outer circumferential surface of the centering section of the ferrule serves as a centering reference surface for the uncoated optical fiber accommodated in the uncoated-fiber holding bore.
The above-described plastic ferrule is fabricated integrally in one unit via an injection molding process or a transfer molding process from resinous material having excellent moldability. With this process, however, there has been conventionally a problem as to how to improve dimensional precision and mechanical strength of a molded product up to a level at which there is no practical problem. In particular, it is required to improve dimensional precision of a ferrule, especially of the centering section thereof (such as an outer-diametral dimensional tolerance, cylindricity, circularity, eccentricity of an uncoated-fiber holding bore relative to an outer-circumferential centering reference surface, etc.), in order to reduce relative eccentricity of the uncoated-fiber holding bores of the centering sections of a pair of ferrules, which are abutted together in the split sleeve, during a connecting process by using the optical connector. In an optical connector used for connection of a single mode optical fiber, for example, extremely high dimensional precision in the order of 0.1 to 1 μm is required for various dimensions of the centering section of the ferrule.
However, in the construction of a conventional plastic ferrule, the thickness of a cylindrical wall of the centering section changes unavoidably from the uncoated-fiber holding bore of a small diameter to the coated-fiber holding bore of a larger diameter. This wall thickness change may cause unbalanced mold shrinkage of resinous material of the ferrule. As a result, in the conventional ferrule, it has been a concern that above-described various dimensional precision, required for the centering section, tends to deteriorate. This deterioration in dimensional precision may result in an increase in relative eccentricity to the counterpart ferrule supported in the split sleeve.
Also, in the conventional plastic ferrule, when the centering section of the counterpart ferrule, abutted in the split sleeve of an optical connector, is made of a hard material such as ceramic, a pressing force (usually in the order of several Newton) exerted upon respective distal end faces of the centering sections of both ferrules during connection may deform the end face of resinous centering section into a concave profile. If this state continues for a long period, it is a concern that plastic deformation (or creep) may take place in the end face of the resinous centering section, which may result in increased optical connection loss of the connector.